Preheat the oven to 425° F. Line 2 muffin tins with paper liners, alternating each to leave space for the muffins to bake. In a bowl, mix the peaches, brown sugar, cinnamon (if using), butter, and vanilla. Spoon 2 teaspoons of the peaches into each muffin cup. Bake 10-15 minutes, until the sauce is bubbling up around the peaches.Meanwhile, make the batter. In a bowl, whisk together the melted butter, brown sugar, yogurt, and eggs. Add the flour, baking powder, and salt. Mix until fully combined. Mix in 1/4 cup to 1/3 cup of the peaches into the batter. Let sit for 5 minutes to allow the batter to rest.Remove the peaches from the oven and divide the batter among muffin tins, filling to the top. Spoon over the remaining peaches, gently swirling them on top of each muffin. If you still have batter in the bowl, you can add 1-2 teaspoons of batter to the top of each muffin. Bake for 8 minutes, turn the oven down to 350° F. and continue baking another 20 minutes, until the center is set and the edges begin to brown. Some of the sugars will caramelize.Let the muffins cool for 5 minutes, and pop them out with a butter knife. Enjoy!!